Texturally-early fluid inclusions in garnets: evidence of the prograde metamorphic path?

Well-formed, texturally-early fluid inclusions in garnets from the Archean Pikwitonei granulite domain, Manitoba, Canada, have been analyzed using microthermometric methods. The mean CO₂ homogenization temperature (to liquid) for inclusions in 12 of 13 samples from the Cauchon Lake-Nelson River area is +15.2° C (n=125, 2σ=8.2° C), corresponding to a CO₂ density of 0.82 g/cm³. Inclusions in the remaining sample have somewhat lower CO₂ homogenization temperatures (mean=+5.4° C, n=24). The studied inclusions contain an estimated 10 to 20 vol. percent H₂O, with minor amounts of other fluid species such as CH₄, N₂, and/or H₂S. The fluid inclusions were probably trapped during early garnet growth at relatively low pressures (≤5 kbar if at 750° C), and appear to have undergone only limited or possibly no subsequent re-equilibration. This interpretation is consistent with the “anti-clock-wise” P-T-t path (heating before loading) determined for the Pikwitonei region by other workers. For such a prograde path, inclusions entrapped early, at high temperatures but at relatively low pressures, would experience internal underpressures during most of the subsequent prograde and retrograde phases of metamorphism. The texturally-early fluid inclusions in garnets from the Pikwitonei region therefore cannot be used to provide direct information about the highest metamorphic temperature and pressure conditions (750° C and 7 kbar). However, the results obtained in this study suggest that texturally-early fluid inclusions in garnets may, in some cases, retain evidence of the prograde metamorphic path.     

Melting experiments on the enstatite-diopside join at 70–224 kbar,including the melting of diopside

 Melting relations on the enstatite−diopside (En, Mg₂Si₂O₆−Di, CaMgSi₂O₆) join, including the compositions of crystalline phases and melts coexisting along the solidi, were experimentally determined in the pressure range 70–224 kbar with a split-sphere anvil apparatus (USSA-2000). Melting is peritectic in enstatite-rich compositions at 70–124 kbar (1840–2100° C) and eutectic at higher pressures, while the diopside-rich clinopyroxene melts azeotropically at 70–165 kbar and up to 300° C lower temperatures than the eutectic. Orthopyroxene is replaced with enstatite-rich clinopyroxene at 120 kbar and 2090°C. First garnet with 17 mol% Di forms on the solidus at 158 kbar and 2100° C. Two garnets coexist on the solidus at 165–183 kbar and 2100° C, garnet coexists with CaSiO₃ perovskite at 183–224 kbar (2100–2230° C) and two coexisting perovskites are stable at higher pressures. The melting curve of diopside was determined at 80–170 kbar; the slope becomes negative at 140 kbar and 2155° C. At 170 kbar and 2100° C, diopside with 96% Di breaks down to garnet with 89% Di and CaSiO₃ perovskite. The new data were used to calculate an improved temperature-pressure phase diagram for the CMAS system, which can be useful for estimating the mineralogy of the Earth's upper mantle. Received: 15 October 1994 / Accepted: 15 October 1995     

Prograde and retrograde history of eclogites from the Eastern Blue Ridge, North Carolina, USA

Abstract The prograde metamorphism of eclogites is typically obscured by chemical equilibration at peak conditions and by partial requilibration during retrograde metamorphism. Eclogites from the Eastern Blue Ridge of North Carolina retain evidence of their prograde path in the form of inclusions preserved in garnet. These eclogites, from the vicinity of Bakersville, North Carolina, USA are primarily comprised of garnet–clinopyroxene–rutile–hornblende–plagioclase–quartz. Quartz, clinopyroxene, hornblende, rutile, epidote, titanite and biotite are found as inclusions in garnet cores. Included hornblende and clinopyroxene are chemically distinct from their matrix counterparts. Thermobarometry of inclusion sets from different garnets record different conditions. Inclusions of clinozoisite, titanite, rutile and quartz (clinozoisite + titanite = grossular + rutile + quartz + H₂O) yield pressures (6–10 kbar, 400–600 °C and 8–12 kbar 450–680 °C) at or below the minimum peak conditions from matrix phases (10–13 kbar at 600–800 °C). Inclusions of hornblende, biotite and quartz give higher pressures (13–16 kbar and 630–660 °C). Early matrix pyroxene is partially or fully broken down to a diopside–plagioclase symplectite, and both garnet and pyroxene are rimmed with plagioclase and hornblende. Hypersthene is found as a minor phase in some diopside + plagioclase symplectites, which suggests retrogression through the granulite facies. Two‐pyroxene thermometry of this assemblage gives a temperature of c. 750 °C. Pairing the most Mg‐rich garnet composition with the assemblage plagioclase–diopside–hypersthene–quartz gives pressures of 14–16 kbar at this temperature. The hornblende–plagioclase–garnet rim–quartz assemblage yields 9–12 kbar and 500–550 °C. The combined P–T data show a clockwise loop from the amphibolite to eclogite to granulite facies, all of which are overprinted by a texturally late amphibolite facies assemblage. This loop provides an unusually complete P–T history of an eclogite, recording events during and following subduction and continental collision in the early Palaeozoic.     

Regional vs contact metamorphism of garnet metapelites in the vicinity of Late Variscan granites (Central Armorican Domain, Brittany, France)

Late Variscan granites intruded Brioverian (Upper Proterozoic) and Lower Paleozoic pelitic sequences to the north of the South Armorican shear zone. In the vicinity of the granites, Brioverian garnet micaschists contain pre/syn-S₂ assemblages with garnet + staurolite and post-S₂ assemblages with staurolite ± andalusite. Andalusite appeared pre/syn- and post-S₂ in garnet-free micaschists. The garnets in the Brioverian micaschists are zoned with increasing Mg and decreasing Mn and Ca from core to inner rim. Only poor garnet zonations occur in Paleozoic hornfelses of enclaves in the Rostrenen granite. The results of a microstructurally controlled application of garnet–biotite geothermometers and garnet–plagioclase geobarometers are similar to P–T trends obtained by the Gibbs method of garnet zonation modelling in the system NCFMnMASH. The P–T paths of a pre/syn-S₂ regional metamorphism are clockwise between 500–550°C/8 kbar and 700°C/5 kbar, followed by cooling decompression. They contrast with isobaric contact metamorphism between 500 and 700°C at 2.5–3 kbar in Paleozoic hornfelses. This points to a two-stage Variscan metamorphism with a pre-granitic pressure-dominated event in the Brioverian micaschists, followed by Late Variscan contact metamorphism, and suggests the existence of a pre-granitic tectonic boundary between the micaschists and overlying low-grade sequences.     

Polymetamorphism and isobaric cooling of pelitic schists from the Annex sulphide deposit, Gordonia Belt, South Africa

Abstract Muscovite-poor pelitic schists in the wallrocks of the Proterozoic Annex sulphide deposit, near Prieska, South Africa, contain peak metamorphic assemblages including Crd + Bt + Sil, St + Sil + Bt, Crd + St + Bt and, rarely, Ky + St ° Crd. All rocks include oligoclase, quartz and commonly Fe–Mn garnet, with or without muscovite. Peak assemblages, assigned to M2 regional metamorphism in the Gordonia Belt (Namaqua Province), are syn- to post-kinematic with respect to the main S2 fabric although larger staurolite grains contain S1 inclusion trails. Garnet–biotite thermometry, utilizing corrections for Fe³⁺, Mn, Al^VI and Ti, yields peak temperatures of 571–624°C at pressures of 4.5–6.0 kbar. Consideration of the sympathetic variation of X_Mn in garnet with X_Mg in biotite and the preserved zoning patterns in prograde garnets, together with the inferred prograde transition from kyanite to sillimanite, indicates that heating occurred during mild decompression to the M2 metamorphic peak. Sillimanite and cordierite grew last in the prograde sequence, possibly related to a pulse of thermal metamorphism (M3) that is found along the margin of the Keimoes Suite batholith to the north. Retrograde assemblages, including Ms + Ky + Chl + Qtz (after Crd + Bt), Ky + Ms (after Sil) and Chl + Ms (after St) indicate a period of isobaric cooling (M4a) terminated by rehydration in the kyanite stability field at about 500°C. The size difference between prograde (1–2-mm) and retrograde (0.05–0.1-mm) mineral grains indicates substantial undercooling below equilibrium positions of relevant retrograde reactions prior to rehydration, and explains why cordierite that grew during M2 is almost completely destroyed. Post-M4a regrowth of staurolite and garnet (M4b) is spatially linked to sites of M4a rehydration. It reached temperatures of 510–530°C, remaining within the stability field of kyanite. A best fit of the observed textural history to the Namaqua orogenic cycle involves collision and heating (M2/D2) followed by granite intrusion (M3), rifting (M4a) and renewed heating due to crustal loading during volcanism (M4b). The P–T path for the Annex region is consistent with those derived from elsewhere in the Gordonia Belt and, with modification, to that published already for the nearby Prieska Copper Mines.     

Origin of garnet in the basic granulites around Saltora,W. Bengal,India

Garnetiferous basic granulites occur, as parts of hornblende-pyroxene- and pyroxene granulites, in a Precambrian terrain around Saltora. The chemistry of the garnetiferous basic granulites is broadly similar to that of the hornblende-pyroxene granulites, their immediate precursors, but in detail they have distinctly higher Fe/Mg ratios. The compositions of the major mafic silicates of the garnetiferous varieties do not reflect higher pressures of formation: the Jd/Ts ratios in calcic pyroxenes are similar to those from the non-garnetiferous varieties, and the pyrope contents of garnets are low. Exchange equilibrium in respect of major elements was established among the mafic silicates in spite of garnets being late overprints. The orthopyroxene — calcic pyroxene pairs from the garnetiferous granulites show lower values of K _D(Mg-Fe) ^opx-cpx than those from the non-garnetiferous granulites, pointing to lower temperature of equilibration. The K _D(Mg-Fe) ^opx-hbl K _D(Mg-Fe) ^cpx-hbl relations show that the more magnesian triads equilibrated at lower temperatures; viewed against experimental data regarding the effect of Mg/Fe ratios on the appearance of garnets in basic rocks, formation of garnets by cooling is strongly indicated. Several intergrowth textures, especially garnet-ilmenite and garnet-quartz (±albite) symplectites, and modal relations argue in favour of composite reactions of the type hornblende+ quartz-→calcic pyroxene+garnet+albite+H₂O, which couple hornblende breakdown reactions with orthopyroxene+anorthite→garnet reactions. The approximate range of pressure and temperature conditions, estimated from experimental data, are 6–8.5 kb and 750–830° C. Since garnets formed by cooling in iron-rich granulites, the garnetiferous granulites do not represent higher pressure subfacies of the granulite facies.     

Petrology and Rare Earth Elements Mineral Chemistry of Chadegan Metabasites (Sanandaj-Sirjan Zone,Iran): Evidence for Eclogite-Facies Metamorphism during Neotethyan Subduction

The metabasites of Chadegan, including eclogite, garnet amphibolite and amphibolite, are forming a part of Sanandaj–Sirjan Zone. These rocks have formed during the subduction of the Neo–Tethys ocean crust under Iranian plate. This subduction resulted in a subduction metamorphism under high pressuremedium temperature of eclogite and amphibolites facies condition. Then the metamorphic rocks were exhumed during the continental collision between the Afro–Arabian continent and the Iranian microcontinent. In the metabasite rocks, with typical MORB composition, garnet preserved a compositional zoning occurred during metamorphism. The magnesium (X_Mg) gradually increases from core to rim of garnets, while the manganese (X_Mn) decreases towards the rim. Chondrite–normalized Rare Earth Element patterns for these garnets exhibit core–to–rim increases in Light Rare Earth Elements. The chondrite–normalized REE patterns of garnets, amphiboles and pyroxenes display positive trend from LREEs to Heavy Rare Earth Elements (especially in garnet), which suggests the role of these minerals as the major controller of HREE distribution. The geochemical features show that the studied eclogite and associated rocks have a MORB origin, and probably formed in a deep–seated subduction channel environment. The geothermometry estimation yields average pressure of ~22 kbar and temperature of 470–520°C for eclogite fomation. The thermobarometry results gave T = 650–700°C and P ≈ 10–11 kbar for amphibolite facies.     

Ca–Mn exchange between grossular and MnCl2 solutions at 2 kbar and 600°°C: reaction mechanism and evidence for non-ideal mixing in spessartine-grossular garnets

 The hydrothermal reaction between grossular and 1 molar manganese chloride solution was studied at 2 kbar and 600 °C at various bulk Ca/(Ca+Mn) compositions: Ca₃Al₂Si₃O₁₂+3Mn²⁺(aq) ⇔ Mn₃Al₂Si₃O₁₂+3Ca²⁺(aq) The reaction products are garnets of the spessartine-grossular solid-solution series which discontinuously armour the dissolving grossular grains. The first garnet to crystallize is spessartine rich (X ^gt _Mn≥0.95), reflecting the high Mn content of the solution, but as the reaction proceeds more calcium-rich garnets progressively overgrow the initial products. The armouring product layer is detached from the dissolving grossular, which allows the progressive overgrowth to occur on both its external and internal surfaces and results in the development of a two directional Ca/(Ca+Mn) zoning pattern in the product grains. The compositional changes in the run products are consistent with attainment of heterogeneous equilibrium between the external rims of the spessartine-grossular garnets and the bulk solutions in runs of duration ≥24 hours. Plots of ln K_D versus X ^gt _Ca maxima show linear variations that are not consistent with the ideal mixing that has been proposed for spessartine-grossular garnets at temperatures of 900 to 1200 °C. The data rather fit a regular solution model with the parameters ΔG° (600 °C, 2 kbar)=−8.0±0.8 kJ/mol and w ^gt _CaMn=2.6±2.0 kJ/mol. Existing solubility measurements and thermodynamic data from other Ca and Mn silicates support the calculated data. Grossular activities calculated using the w ^gt _CaMn parameter indicate that even in manganese-rich metapelites pressure estimates calculated using the garnet-plagioclase-Al₂SiO₅-quartz barometer will not be increased by more than 0.2 kbar. Received: 18 January 1995/Accepted: 4 June 1996     

High-pressure eclogites in northern Jiangsu – southern Shandong province, eastern China

Abstract Eclogites are distributed for more than 500 km along a major tectonic boundary between the Sino-Korean and Yangtze cratons in central and eastern China. These eclogites usually have high-P assemblages including omphacite + kyanite and/or coesite (or its pseudomorph), and form a high-P eclogite terrane. They occur as isolated lenses or blocks 10 cm to 300 m long in gneisses (Type I), serpentinized garnet peridotites (Type II) and marbles (Type III). Type I eclogites were formed by prograde metamorphism, and their primary metamorphic mineral assemblage consists mainly of garnet [pyrope (Prp) = 15–40 mol%], omphacite [jadeite (Jd) = 34–64 mol%], pargasitic amphibole, kyanite, phengitic muscovite, zoisite, an SiO₂ phase, apatite, rutile and zircon. Type II eclogites characteristically contain no SiO₂ phase, and are divided into prograde eclogites and mantle-derived eclogites. The prograde eclogites of Type II are petrographically similar to Type I eclogites. The mantle-derived eclogites have high MgO/(FeO + Fe₂O₃) and Cr₂O₃ compositions in bulk rock and minerals, and consist mainly of pyrope-rich garnet (Prp = 48–60 mol%), sodic augite (Jd = 10–27 mol%) and rutile. Type III eclogites have an unusual mineral assemblage of grossular-rich (Grs = 57 mol%) garnet + omphacite (Jd = 30–34 mol%) + pargasite + rutile. Pargasitic and taramitic amphiboles, calcic plagioclase (An₆₈), epidote, zoisite, K-feldspar and paragonite occur as inclusions in garnet and omphacite in the prograde eclogites. This suggests that the prograde eclogites were formed by recrystallization of epidote amphibolite and/or amphibolite facies rocks with near-isothermal compression reflecting crustal thickening during continent–continent collision of late Proterozoic age. Equilibrium conditions of the prograde eclogites range from P > 26 kbar and T= 500–750°C in the western part to P > 28 kbar and T= 810–880°C in the eastern part of the high-P eclogite terrane. The prograde eclogites in the eastern part are considered to have been derived from a deeper position than those in the western part. Subsequent reactions, manifested by (1) narrow rims of sodic plagioclase or paragonite on kyanite and (2) symplectites between omphacite and quartz are interpreted as an effect of near-isothermal decompression during the retrograde stage. The conditions at which symplectites re-equilibrated tend to increase from west (P < 10 kbar and T < 580°C) to east (P > 9 kbar and T > 680°C). Equilibrium temperatures of Type II mantle-derived eclogites and Type III eclogite are 730–750°C and 680°C, respectively.     

Pressure-temperature Evolution of the Metapelites in the Motuo Area,the Eastern Himalayan Syntaxis

The Motuo area is located in the east of the Eastern Himalayan Syntaxis. There outcrops a sequence of high-grade metamorphic rocks, such as metapelites. Petrology and mineralogy data suggest that these rocks have experienced three stages of metamorphism. The prograde metamorphic mineral assemblages(M1) are mineral inclusions(biotite + plagioclase + quartz ± sillimanite ± Fe-Ti oxides) preserved in garnet porphyroblasts, and the peak metamorphic assemblages(M2) are represented by garnet with the lowest XSps values and the lowest XFe# ratios and the matrix minerals(plagioclase + quartz ± Kfeldspar + biotite + muscovite + kyanite ± sillimanite), whereas the retrograde assemblages(M3) are composed of biotite + plagioclase + quartz symplectites rimming the garnet porphyroblasts. Thermobarometric computation shows that the metamorphic conditions are 562–714°C at 7.3–7.4 kbar for the M1 stage, 661–800°C at 9.4–11.6 kbar for the M2 stage, and 579–713°C at 5.5–6.6 kbar for the M3 stage. These rocks are deciphered to have undergone metamorphism characterized by clockwise P-T paths involving nearly isothermal decompression(ITD) segments, which is inferred to be related to the collision of the India and Eurasia plates.     

Eclogites and polyphase P–T cycling in the Caledonian Uppermost Allochthon in Troms, northern Norway

Eclogites in the Tromsø area, northern Norway, are intimately associated with meta-supracrustals within the Uppermost Allochthon of the Scandinavian Caledonides (the Tromsø Nappe Complex). The whole sequence, which includes pelitic to semipelitic schists and gneisses, marbles and calc-silicate rocks, quartzofeldspathic gneisses, metabasites and ultramafites, has undergone three main deformational/metamorphic events (D₁/M₁, D₂/M₂ and D₃/M₃). Detailed structural, microtextural and mineral chemical studies have made it possible to construct separate P–T paths for these three events. Chemically zoned late syn- to post-D₁ garnets with inclusions of Bt, Pl and Qtz in Ky-bearing metapelites indicate a prograde evolution from 636°C, 12.48 kbar to c. 720°C, 14–15 kbar. This latter result is in agreement with Grt–Cpx geothermometry and Grt–Cpx–Pl–Qtz geobarometry on eclogites and trondhjemitic to dioritic gneisses. Maximum pressures at c. 675°C probably reached 17–18 kbar based on Cpx–Pl–Qtz inclusions in eclogitic garnets, and Grt–Ky–Pl–Qtz and Jd–Ab–Qtz in trondhjemitic gneisses. Post-D₁/pre-D₂ decompressional breakdown of the high-P assemblages indicates a substantial drop in pressure at this stage. Inclusions and chemical zoning in syn- to post-D₂ garnets from metapelites record a second episode of prograde metamorphism, from 552°C, 7.95 kbar, passing through a maximum pressure of 10.64 kbar at 644°C, with final equilibration at c. 665°C, 9–10 kbar. The corresponding apparently co-facial paragenesis Grt + Cpx + Pl + Qtz in metabasites yields c. 635°C, 8–10 kbar. In the metapelites post-D₃, Grt in apparent equilibrium with Bt, Phe and Pl yield c. 630°C, 9 kbar. The D₁/M₁ and D₂/M₂ episodes are exclusively recorded in the Tromsø Nappe Complex and must thus pre-date the emplacement of this allochthonous unit on top of the underlying Lyngen Nappe, while the D₃/M₃ episode is common for the two units. A previously published Sm–Nd mineral isochron (Grt–Cpx–Am) on a partly retrograded and recrystallized ecologite of 598 ± 107 Ma represents either the timing of formation of the eclogites or the post-eclogite/pre-D₂ decompression stage, while a Rb–Sr whole rock isochron of an apparently post-D₁/pre-D₂ granite of 433 ± 11 Ma is consistent with a K–Ar age of post-D₁/pre-D₂ amphiboles from a retrograded eclogite of 437 ± 16 Ma which most likely record cooling below the 475–500°C isotherm after the M₃ metamorphism.     

Metamorphic history of eclogites and country rock gneisses in the Aktyuz area,Northern Tien‐Shan,Kyrgyzstan: a record from initiation of subduction through to oceanic closure by continent–continent collision

Eclogites and related high‐P metamorphic rocks occur in the Zaili Range of the Northern Kyrgyz Tien‐Shan (Tianshan) Mountains, which are located in the south‐western segment of the Central Asian Orogenic Belt. Eclogites are preserved in the cores of garnet amphibolites and amphibolites that occur in the Aktyuz area as boudins and layers (up to 2000 m in length) within country rock gneisses. The textures and mineral chemistry of the Aktyuz eclogites, garnet amphibolites and country rock gneisses record three distinct metamorphic events (M1–M3). In the eclogites, the first MP–HT metamorphic event (M1) of amphibolite/epidote‐amphibolite facies conditions (560–650 °C, 4–10 kbar) is established from relict mineral assemblages of polyphase inclusions in the cores and mantles of garnet, i.e. Mg‐taramite + Fe‐staurolite + paragonite ± oligoclase (An_<16) ± hematite. The eclogites also record the second HP‐LT metamorphism (M2) with a prograde stage passing through epidote‐blueschist facies conditions (330–570 °C, 8–16 kbar) to peak metamorphism in the eclogite facies (550–660 °C, 21–23 kbar) and subsequent retrograde metamorphism to epidote‐amphibolite facies conditions (545–565 °C and 10–11 kbar) that defines a clockwise P–T path. thermocalc (average P–T mode) calculations and other geothermobarometers have been applied for the estimation of P–T conditions. M3 is inferred from the garnet amphibolites and country rock gneisses. Garnet amphibolites that underwent this pervasive HP–HT metamorphism after the eclogite facies equilibrium have a peak metamorphic assemblage of garnet and pargasite. The prograde and peak metamorphic conditions of the garnet amphibolites are estimated to be 600–640 °C; 11–12 kbar and 675–735 °C and 14–15 kbar, respectively. Inclusion phases in porphyroblastic plagioclase in the country rock gneisses suggest a prograde stage of the epidote‐amphibolite facies (477 °C and 10 kbar). The peak mineral assemblage of the country rock gneisses of garnet, plagioclase (An_11–16), phengite, biotite, quartz and rutile indicate 635–745 °C and 13–15 kbar. The P–T conditions estimated for the prograde, peak and retrograde stages in garnet amphibolite and country rock are similar, implying that the third metamorphic event in the garnet amphibolites was correlated with the metamorphism in the country rock gneisses. The eclogites also show evidence of the third metamorphic event with development of the prograde mineral assemblage pargasite, oligoclase and biotite after the retrograde epidote‐amphibolite facies metamorphism. The three metamorphic events occurred in distinct tectonic settings: (i) metamorphism along the hot hangingwall at the inception of subduction, (ii) subsequent subduction zone metamorphism of the oceanic plate and exhumation, and (iii) continent–continent collision and exhumation of the entire metamorphic sequences. These tectonic processes document the initial stage of closure of a palaeo‐ocean subduction to its completion by continent–continent collision.     

Petrology of eclogites from north of Shahrekord, Sanandaj-Sirjan Zone, Iran

Summary Metabasic rocks were recently found within a ductile shear zone in the north of Shahrekord, being a part of the structural zone of Sanandaj-Sirjan, SW Iran. The rocks give evidence of a so far unrecognized eclogite facies metamorphic event and testify to high pressure metamorphism in the Sanandaj-Sirjan Zone, near the Main Zagros Reverse Fault, which is the assumed suture zone between the Arabian plate and the Iranian block. The eclogites occur as lenses or blocks within ortho- and paragneisses. The petrographic features and reaction textures display at least two main metamorphic stages: (1) a peak eclogite facies stage, and (2) a subsequent amphibolite facies stage. The eclogite facies metamorphism is indicated by omphacite + garnet + sodic-calcic amphiboles (barroisite, magnesiokatophorite and magnesiotaramite) + phengite + rutile + (clino-)zoisite + quartz ± dolomite. The garnets are mainly almandine-rich, which fits with the C-type eclogite classification. Calcic amphiboles (hornblende, tschermakite and pargasite) + plagioclase are secondary phases formed during the retrograde amphibolite-facies metamorphism. P-T estimates for the eclogite facies give pressures of 21–24 kbar and temperatures of 590–630 °C (geothermometry) and 470–520 °C (THERMOCALC), respectively. Geothermobarometry for the amphibolite-facies metamorphism yields 10–11 kbar and 650–700 °C. Author’s address: Ali Reza Davoudian, Department of Natural Resources, Shahrekord University, Shahrekord, Iran     

Phase equilibria modelling and LASS monazite petrochronology: P–T–t constraints on the evolution of the Priest River core complex,northern Idaho

Phase equilibria modelling, laser‐ablation split‐stream (LASS)‐ICP‐MS petrochronology and garnet trace‐element geochemistry are integrated to constrain the P–T–t history of the footwall of the Priest River metamorphic core complex, northern Idaho. Metapelitic, migmatitic gneisses of the Hauser Lake Gneiss contain the peak assemblage garnet + sillimanite + biotite ± muscovite + plagioclase + K‐feldspar ± rutile ± ilmenite + quartz. Interpreted P–T paths predict maximum pressures and peak metamorphic temperatures of ~9.6–10.3 kbar and ~785–790 °C. Monazite and xenotime ²⁰⁸Pb/²³²Th dates from porphyroblast inclusions indicate that metamorphism occurred at c. 74–54 Ma. Dates from HREE‐depleted monazite formed during prograde growth constrain peak metamorphism at c. 64 Ma near the centre of the complex, while dates from HREE‐enriched monazite constrain the timing of garnet breakdown during near‐isothermal decompression at c. 60–57 Ma. Near‐isothermal decompression to ~5.0–4.4 kbar was followed by cooling and further decompression. The youngest, HREE‐enriched monazite records leucosome crystallization at mid‐crustal levels c. 54–44 Ma. The northernmost sample records regional metamorphism during the emplacement of the Selkirk igneous complex (c. 94–81 Ma), Cretaceous–Tertiary metamorphism and limited Eocene exhumation. Similarities between the Priest River complex and other complexes of the northern North American Cordillera suggest shared regional metamorphic and exhumation histories; however, in contrast to complexes to the north, the Priest River contains less partial melt and no evidence for diapiric exhumation. Improved constraints on metamorphism, deformation, anatexis and exhumation provide greater insight into the initiation and evolution of metamorphic core complexes in the northern Cordillera, and in similar tectonic settings elsewhere.     

Phase relationships in granulitic metapelites from the Ivrea-Verbano zone (Northern Italy)

Paragneisses of the Ivrea-Verbano zone exhibit over a horizontal distance of 5 km mineralogical changes indicative of the transition from amphibolite to granulite facies metamorphism. The most obvious change is the progressive replacement of biotite by garnet via the reaction: a $${\text{Biotite + sillimanite + quartz }} \to {\text{ Garnet + K - feldspar + H}}_{\text{2}} {\text{O}}$$ which results in a systematic increase in the modal ratio g = (garnet)/(garnet + biotite) with increasing grade. The systematic variations in garnet and biotite contents of metapelites are also reflected by the compositions of these phases, both of which become more magnesian with increasing metamorphic grade. The pressure of metamorphism has been estimated from the Ca₃Al₂Si₃O₁₂ contents of garnets coexisting with plagioclase, sillimanite and quartz. These phases are related by the equilibrium: b $$\begin{gathered} 3 CaAl_2 {\text{Si}}_{\text{2}} {\text{O}}_{\text{8}} \rightleftharpoons Ca_3 Al_2 {\text{Si}}_{\text{3}} {\text{O}}_{{\text{12}}} + 2 Al_2 {\text{SiO}}_{\text{5}} + {\text{SiO}}_{\text{2}} \hfill \\ plagioclase garnet sillimanite quartz \hfill \\ \end{gathered} $$ which has been applied to these rocks using the available data on the mixing properties of plagioclase and garnet solid solutions. Temperature and f _{H 2O} estimates have been made in a similar way using thermodynamic data on the biotite-garnet reaction (a) and the approximate solidus temperatures of paragneisses. Amphibolite to granulite facies metamorphism in the Ivrea-Verbano zone took place in the P-T ranges 9–11 kb and 700–820 °C. The differences in temperature and pressure of metamorphism between g= 0 and g = 1 (5 kms horizontal distance) were less than 50° C and approximately 1 kb. Retrogression and re-equilibration of garnets and biotites in the metapelites extended to temperatures more than 50° C below and pressures more than 1.5 kb below the peak of metamorphism, the degree of retrogression increasing with decreasing grade of the metamorphic “peak”. The pressure and temperature of the peak of metamorphism are not inconsistent with the hypothesis that the Ivrea-Verbano zone is a slice of upthrusted lower crust from the crust-mantle transition region, although it appears that the thermal gradient was too low for the zone to represent a near-vertical section through the crust. The most reasonable explanation of the granulite facies metamorphism is that it arose through intrusion of mafic rocks into a region already undergoing recrystallisation under amphibolite facies conditions.     

Origin of atoll garnet in schists from the Alpuj��rride Complex (Central zone of the Betic Cordillera, Spain): Implications on the P-T evolution

Atoll-shaped and normal garnets from schists of the Betic Cordillera (Spain) were studied by electron microprobe, scanning electron microscopy and microstructural electron backscattered scanning diffraction (EBSD). Medium-grade schists contain a textural variety of atoll garnets, characterized by the presence of muscovite, annite, and quartz as main ??core?? phases. Zoning patterns, EBSD analyses and themobarometric data indicate that the micaceous intergrowths formed in most cases through breakdown of a first garnet generation, with orientation of micas being controlled by garnet (c*_mica//[111]*_Grt and c*_mica//[110]*_Grt as main crystallographic relationships). Rings formed from multiple nucleation and coalescence, with orientation being controlled, in some cases, by the relics of the initial garnet and more generally by mica orientation. P-T estimates indicate that the first stage of garnet growth occurred at relatively high P (9?C12?kbar/500?C550°C) whereas the second metamorphic stage occurred at lower P and slightly higher T conditions (5?C7?kbar/500?C600°C).     

High pressure single-crystal synthesis, structure and compressibility of the garnet Mn2+ 3Mn3+ 2[SiO4]3

Single crystals of the garnet Mn²⁺ ₃Mn³⁺ ₂[SiO₄]₃ and coesite were synthesised from MnO₂-SiO₂ oxide mixtures at 1000°C and 9 GPa in a multianvil press. The crystal structure of the garnet [space group Ia3¯d, a=11.801(2) Å] was refined at room temperature and 100 K from single-crystal X-ray data to R1=2.36% and R1=2.71%, respectively. In contrast to tetragonal Ca₃Mn³⁺ ₂[GeO₄]₃ (space group I4₁/a), the high-pressure garnet is cubic and does not display an ordered Jahn-Teller distortion of octahedral Mn³⁺. A disordered Jahn-Teller distortion either dynamic or static is evidenced by unusual high anisotropic displacement parameters. The room temperature structure is characterised by following bond lengths: Si-O=1.636(4) Å (tetrahedron), Mn³⁺-O=1.995 (4) Å (octahedron), Mn²⁺-O=2.280(5) and 2.409(4) Å (dodecahedron). The cubic structure was preserved upon cooling to 100 K [a=11.788(2) Å] and upon compressing up to 11.8 GPa in a diamond-anvil cell. Pressure variation of the unit cell parameter expressed by a third-order Birch-Murnaghan equation of state led to a bulk modulus K ₀=151.6(8) GPa and its pressure derivatives K′=6.38(19). The peak positions of the Raman spectrum recorded for Mn²⁺ ₃Mn³⁺ ₂[SiO₄]₃ were assigned based on a calderite Mn²⁺ ₃Fe³⁺ ₂[SiO₄]₃ model extrapolated from andradite and grossular literature data.     

High-pressure granulites in the Sanggan area, North China craton: metamorphic evolution, P–T paths and geotectonic significance

High‐pressure basic granulites are widely distributed as enclaves and sheet‐like blocks in the Huaian TTG gneiss terrane in the Sanggan area of the Central Zone of the North China craton. Four stages of the metamorphic history have been recognised in mineral assemblages based on inclusion, exsolution and reaction textures integrated with garnet zonation patterns as revealed by compositional maps and compositional profiles. The P–T conditions for each metamorphic stage were obtained using thermodynamically and experimentally calibrated geothermobarometers. The low‐Ca core of growth‐zoned garnet, along with inclusion minerals, defines a prograde assemblage (M1) of garnet + clinopyroxene + plagioclase + quartz, yielding 700 °C and 10 kbar. The peak of metamorphism at about 750–870 °C and 11–14.5 kbar (M2) is defined by high‐Ca domains in garnet interiors and inclusion minerals of clinopyroxene, plagioclase and quartz. Kelyphites or coronas of orthopyroxene + plagioclase ± magnetite around garnet porphyroblasts indicate garnet breakdown reactions (M3) at conditions around 770–830 °C and 8.5–10.5 kbar. Garnet exsolution lamellae in clinopyroxene and kelyphites of amphibole + plagioclase around garnet formed during the cooling process at about 500–650 °C and 5.5–8 kbar (M4). These results help define a sequential P–T path containing prograde, near‐isothermal decompression (ITD) and near‐isobaric cooling (IBC) stages. The clockwise hybrid ITD and IBC P–T paths of the HP granulites in the Sanggan area imply a model of thickening followed by extension in a collisional environment. Furthermore, the relatively high‐pressures (6–14.5 kbar) of the four metamorphic stages and the geometry of the P–T paths suggest that the HP granulites, together with their host Huaian TTG gneisses, represent the lower plate in a crust thickened during collision. The corresponding upper‐plate might be the tectonically overlying Khondalite series, which was subjected to medium‐ to low‐pressure (MP/LP: 7–4 kbar) granulite facies metamorphism with a clockwise P–T path including an ITD segment. Both the HP and the MP/LP granulite facies events occurred contemporaneously at c. 1.90–1.85 Ga in a collisional environment created by the assembly process of the North China craton.     

Dehydration melting of tonalites. Part I. Beginning of melting

 The beginning of dehydration melting in the tonalite system (biotite-plagioclase-quartz) is investigated in the pressure range of 2–12 kbar. A special method consisting of surrounding a crystal of natural plagioclase (An₄₅) with a biotite-quartz mixture, and observing reactions at the plagioclase margin was employed for precise determination of the solidus for dehydration melting. The beginning of dehydration melting was worked out at 5 kbar for a range of compositions of biotite varying from iron-free phlogopite to iron-rich Ann₇₀, with and without titanium, fluorine and extra aluminium in the biotite. The dehydration melting of phlogopite + plagioclase (An₄₅) + quartz begins between 750 and 770°C at pressures of 2 and 5 kbar, at approximately 740°C at 8 kbar and between 700 and 730°C at 10 kbar. At 12 kbar, the first melts are observed at temperatures as low as 700°C. The data indicate an almost vertical dehydration melting solidus curve at low pressures which bends backward to lower temperatures at higher pressures (> 5 kbar). The new phases observed at pressures ≤ 10 kbar are melt + enstatite + clinopyroxene + potassium feldspar ± amphibole. In addition to these, zoisite was also observed at 12 kbar. With increasing temperature, phlogopite becomes enriched in aluminium and deficient in potassium. Substitution of octahedral magnesium by aluminium and titanium in the phlogopite, as well as substitution of hydroxyl by fluorine, have little effect on the beginning of dehydration melting temperatures in this system. The dehydration melting of biotite (Ann₅₀) + plagioclase (An₄₅) + quartz begins 50°C below that of phlogopite bearing starting composition. Solid reaction products are orthopyroxene + clinopyroxene + potassium feldspar ± amphibole. Epidote was also observed above 8 kbar, and garnet at 12 kbar (750°C). The experiments on the iron-bearing system performed at ≤ 5 kbar were buffered with NiNiO. The f _{O 2} in high pressure runs lies close to CoCoO. With the substitution of octahedral magnesium and iron by aluminium and titanium, and replacement of hydroxyl by fluorine in biotite, the beginning of dehydration melting temperatures in this system increase up to 780°C at 5 kbar, which is 70°C above the beginning of dehydration melting of the assemblage containing biotite (Ann₅₀) of ideal composition. The dehydration melting at 5 kbar in the more iron-rich Ann₇₀-bearing starting composition begins at 730°C, and in the Ann₂₅-bearing assemblage at 710°C. This indicates that quartz-biotite-plagioclase assemblages with intermediate compositions of biotite (Ann₂₅ and Ann₅₀) melt at lower temperatures as compared to those containing Fe-richer or Mg-richer biotites. This study shows that the dehydration melting of tonalites may begin at considerably lower temperatures than previously thought, especially at high pressures (>5 kbar). Received: 27 December 1995 / Accepted: 7 May 1996     

Thermodynamics of minerals of variable composition: Andradite-grossularite and pistacite-clinozoisite solid solutions

Exchange-mineral equilibria with Al and Fe³⁺ aqueous chloride solutions (aq.), Andr + AlCl _aq ³ = FeCl _aq ³ + Gros, (1) Psc + AlCl _aq ³ = FeCl _aq ³ + Czo, (2) were studied under the following experimental conditions: 500°;C and 580°;C and 1 and 2 kbar, respectively, with an overall concentration of metals in the aqueous solutions of about 0.5 M and pH 3. The mixing functions of the components in garnet and epidote were calculated from the experimental data. Thermodynamic treatment of experimental evidence for reaction (1) led to the conclusion that, within the accuracy of experiment, garnet in the andradite-grossularite series was an ideal solid solution. However, epidote solid solution markedly departed from the ideal, as was shown by concentration and pressure-temperature (PT) dependencies of Gibbs's molar excess energies and by mixing-volume concentration dependence.